The present invention relates generally to a system and method for controlling the flow of fluids through a conduit, and in particular to controlling a pump acting on a conduit for increasing the uniformity of the fluid flow through the conduit.
In certain systems used for infusing parenteral fluids intravenously to a patient, a pumping mechanism engages a length of conduit or tubing of a flexible administration set to pump the parenteral fluid to the patient at a selected flow rate. A peristaltic pump is one commonly used type of pumping mechanism and employs the sequential occlusion of the administration set tubing to move the fluid through the tubing to the patient.
Linear-type peristaltic pumps typically include a row of adjacent, reciprocating pumping fingers that are sequentially urged against the fluid administration set tubing to occlude adjacent segments of that tubing in a wave-like action to force fluid through the tubing. The reciprocating, sequential motion of the fingers is accomplished in one arrangement by the use of a cam shaft rotated by a drive motor. Disposed along the length of the cam shaft are a plurality of adjacent cams having generally symmetrical lobe geometries with one cam operating each finger. The cams are disposed along the cam shaft so that adjacent lobes project at different angular positions relative to the cam shaft. The fingers in turn advance and retract sequentially in accordance with the angular positions of the respective cam lobes and rotation of the cam shaft.
The drive motor typically comprises a step motor having a certain number of motor steps per complete rotation of its armature; for example, two-hundred steps per 360 degrees of rotation. Typically, a pump cycle is defined as a complete cycle of the pumping mechanism. For example, in the case of a twelve-finger linear peristaltic pump, a pump cycle is complete when all twelve fingers have engaged the fluid conduit and returned to the positions they had at the start of the cycle. In many such systems, when the pump mechanism has completed a full cycle the step motor will have also traveled through 360 degrees of rotation, thereby causing it to have travelled through all of its steps in that rotation.
Each incremental movement of the motor causes a corresponding incremental movement of the cams and fingers and results in a discrete volume of fluid or "step volume" being pumped through the conduit. An inherent characteristic of linear peristaltic pumps is that step volumes vary from other step to step, and at certain points over a pump cycle the step volume may even be negative (i.e., reverse flow). This reverse flow results when the outlet side fingers of the linear peristaltic pump are retracted from the tubing and a reverse flow surge backfills the tubing pumping segment due to a pressure difference between the pumping segment and the downstream segment.
In one effort to increase the flow uniformity within a peristaltic pump cycle, the design of the pumping mechanism was tailored. For example, tailored, non-symmetrical cam lobes have been developed to accelerate, decelerate or limit the advancement of the pumping fingers as they engage and disengage segments of the tubing. Some of these designs have resulted in increased uniformity of volumes pumped per motor step at a particular design flow rate. However, it has been found that the effectiveness of these designs decreases at flow rates that differ significantly from the design flow rate.
Another approach to increasing flow uniformity is described in U.S. Pat. No. 5,716,194 to Butterfield et al., entitled SYSTEM FOR INCREASING FLOW UNIFORMITY, the contents of which are incorporated herein by reference. In U.S. Pat. No. 5,716,194, flow uniformity was enhanced by grouping several adjacent steps into larger "supersteps," with each superstep comprised of a group of steps. By carefully grouping of the steps, supersteps can be created in such a way that each superstep has essentially the same volume of fluid as the other supersteps. For example, one superstep may consist of 7 relatively low-volume motor steps, while another superstep may consist of 3 larger-volume motor steps. By associating more of the low-volume motor steps on the first superstep, the total volume of the first superstep approximately equals the total volume of the second superstep. With supersteps of generally equal volume and period, flow uniformity is enhanced.
For lower flow rates, the use of such supersteps can require long pauses in pump operation between the steps. A single motor step may, for example, produce a bolus of fluid which, to produce flow at the desired flow rate, requires substantial time to elapse before the next motor step occurs. Moreover, in some cases, even with long pauses between steps, a particularly large-volume step may cause the system to momentarily exceed the desired flow rate. The problem of such large-volume steps could be increased by the use of supersteps that consist of more than one step.
Various modifications to fluid pump drive systems can be made to address uniformity at low flow rates, including the addition of a gear train and/or development of a pump having a greater number of steps per revolution. Such modifications can, however, be expensive in that they typically require development of an entirely new pump mechanism.
In part to address concerns for low flow rates, a motor drive technique known as "microstepping" was developed, wherein each motor step was subdivided into a series of smaller microsteps. For example, each motor step might be subdivided into up to eight different microsteps. Those microsteps could then be grouped into "packets" of microsteps, with each packet having essentially the same volume as other packets.
Microstepping has been found to increase flow uniformity and significantly reduce motor noise. Microstepping involves driving the step motor through a step with a series of current magnitude states that generate small angular displacements of the field vector position. The sum of these displacements equals that of one step. Because instantaneous torque is approximately a sinusoidal function of angular displacement of a motor's field vector position from its rotor position, a smaller angular displacement results in a lower instantaneous torque. A lower instantaneous torque generates an angular acceleration at the leading edge of each "microstep" smaller than that generated at the leading edge of each step in "full step" drive mode. The effect is to spread the large acceleration that normally occurs at the beginning of a step over the entire step as a series of small accelerations, thus reducing the level of acoustic noise. Thus, rather than turning through an entire step in near-instantaneous fashion, the motor can instead moves through a series of distinct incremental microsteps, each of which involves only a portion of the movement turn of an entire step.
Several existing systems make use of microsteps in various drive motors, including fluid pump motors. For example, U.S. patent application Ser. No. 08/526,468 to Holdaway, entitled "OPEN-LOOP STEP MOTOR CONTROL SYSTEM," which is incorporated herein by reference in its entirety, describes using microsteps in driving an infusion pump step motor.
In existing implementations, the duration of each microstep was typically fixed at a nominal value, such as 2.36 milliseconds. An entire packet of microsteps would often be made in relatively rapid succession, followed by a "non-flow time" during which no motor movement would occur. The average flowrate was adjusted by reducing or increasing the volume in the packets (i.e., by adjusting the number of microsteps in each packets), and also by adjusting the non-flow time (i.e., the time between microsteps in which the motor was not moving).
The non-flow period could be actively varied in order to change the average flow rate as well as to enhance other system functions. For example, U.S. patent application Ser. No. 08/688,698 to Butterfield, entitled FLUID FLOW RESISTANCE MONITORING SYSTEM, which is incorporated herein by reference in its entirety, describes a system that varies fluid delivery, including non-flow periods, using a pseudorandom code. For very low flow rates, the non-flow time might become relatively large. For example, a desired flow rate of 0.1 ml/hr might involve a non-flow period on the order of 200 seconds.
In fluid driving systems, there are circumstances wherein maximum flow uniformity is desirable. For example, in parenteral infusion of some fluids that require very low flow rates, such as certain fast acting (i.e., short half-life) drugs, it can be desirable to maintain minimal fluctuation of the instantaneous flow rate. This need for minimal fluctuation of the flow rate can become most acute in the lower ranges of flow typically produced by commercial peristaltic infusion devices, such as the range from 0.1 to 1.0 ml/hr.
Some organizations, such as the Emergency Care Research Institute (ECRI), have promulgated ratings of flow uniformity based on the interval between "flow steps" at the lowest flow rate achieved. Such ratings, although typically somewhat indefinite, can provide useful guidelines. For example, ECRI rates an infusion pumps flow uniformity as "excellent" if less than 20 seconds elapse between "flow steps" at the "lowest rate programmable." Assuming that the ECRI rating is based on having steps of equal volume, many current commercial devices are far from meeting such criteria.
Hence those skilled in the art have recognized the need for increasing flow uniformity, particularly at low flow rates. The present invention fulfills these needs and others.